Golf ball

ABSTRACT

A golf ball with improved spin performance, i.e., controllability with an iron, as well as improved resilience and scratch resistance is provided. A mixture of polyurethane-type thermoplastic elastomer and a metal compound (other than titanate oxide), for instance, zinc oxide or magnesium oxide, is used as a cover material of the golf ball. More preferably, the metal compound is mixed at 5 to 15 parts by weight for 100 parts by weight of polyurethane-type thermoplastic elastomer, and the Shore D hardness of the cover material of the golf ball is in the range of 30 to 60.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a golf ball that has good spin performance, i.e., that allows easy control of spin rate with an iron, and that has excellent scratch resistance as well as superior resilience.

[0003] 2. Description of the Background Art

[0004] Conventionally, the golf balls which are produced by forming a thread-wound layer on a liquid center and covering thereon a balata cover have been widely used among advanced golf players and professional golf players for its excellent shot feel and controllability. The structure of such a golf ball, however, involves complex manufacturing steps and has inferior cut resistance so that various soft cover materials that replace the balata cover have been proposed in the recent years.

[0005] For instance, in Japanese Patent Laying-Open No. 10-179802, a golf ball is proposed which is characterized in that the base resin for the cover is composed of a heated mixture, as the main component, of two components of ionomer resin and styrene-butadiene-styrene block copolymer having polybutadiene block containing epoxy group or styrene-isoprene-styrene block copolymer having polyisoprene block containing epoxy group, that the modulus of flexural rigidity of the composition forming the cover is between 50 and 300 MPa, and that the Shore D hardness is between 40 and 60. Such technique intends to improve the shot feel, the spin performance, the flying performance, and the cut resistance.

[0006] Moreover, U.S. Pat. No. 5,716,293 discloses improving the shot feel as well as increasing the spin rate by a short iron by using a solid rubber center containing an oily substance and a soft cover material. An oil-resistant rubber and an ionomer resin of high hardness are used on the outside of the solid rubber center, however, so that there still is room for improvement with respect to the resilience and the shot feel.

[0007] Furthermore, Japanese Patent Laying-Open No. 2000-176050 discloses a technique in which the product obtained by heating and mixing polyester-type thermoplastic elastomer with a metal compound is used as a golf ball material, particularly for an intermediate layer, to improve the feeling and the carry of the golf ball.

SUMMARY OF THE INVENTION

[0008] The present invention is made to solve such problems of a conventional golf ball as described above. The object of the present invention is to provide a golf ball that has good spin performance, i.e., that allows easy control of spin rate with an iron, and that has excellent resilience as well as superior scratch resistance.

[0009] The present invention is a golf ball comprising a core and a cover in which a material of the cover is obtained by mixing polyurethane-type thermoplastic elastomer with a metal compound (other than titanate oxide). In the case in which the cover consists of multiple layers, said material forms a material for a cover of at least one layer.

[0010] More preferably, the present invention is a golf ball in which a material obtained by mixing polyurethane-type thermoplastic elastomer at 100 parts by weight with a metal compound (other than titanate oxide) at 5 to 15 parts by weight forms a material for a cover of at least one layer.

[0011] Preferably, the metal compound is an oxide or a hydroxide of a metal selected from the group consisting of zinc, sodium, magnesium, lithium, neodymium, copper, calcium, aluminum, iron, and barium. In particular, it is preferred that the metal compound is zinc oxide. Further, it is desirable that a hardness of the cover covering a core is 30 to 60 in Shore D hardness.

[0012] The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] The present invention is a golf ball formed by a core and a cover that covers the core.

[0014] The polyurethane-type thermoplastic elastomer used for the cover of the golf ball according to the present invention is a polymer containing as the main chain the urethane bonding produced by aromatic diisocyanate, aliphatic diisocyanate, or alicyclic diisocyanate and polyol having a polyester structure or polyol having a polyether structure.

[0015] The polyurethane-type thermoplastic elastomer has a basic structure shown in formulas (5) and (7). In these formulas, the one whose diol component (Y in the formulas) forming a soft segment is polyester polyol is polyester-type polyurethane thermoplastic elastomer, while the one whose diol component (Y in the formulas) forming a soft segment is polyether glycol is polyether-type polyurethane thermoplastic elastomer.

[0016] Such polyurethane-type thermoplastic elastomer produces urethane prepolymer shown in formula (3) by having diisocyanate shown in formula (1) react with diol shown in formula (2). Then, the urethane prepolymer is made to react with diol of low molecular weight shown in formula (4) as a crosslinking agent to produce polyurethane-type thermoplastic elastomer of formula (5). On the other hand, urethane prepolymer of formula (3) may be made to react with diamine of low molecular weight shown in formula (6) to produce polyurethane-type thermoplastic elastomer having an urea bonding within molecules of formula (7).

[0017] Throughout the formulas, X indicates aliphatic, aromatic, and alicyclic hydrocarbon, Y indicates polyester or polyol residue, and Z indicates aromatic or aliphatic hydrocarbon.

[0018] Specific compounds of diisocyanate include 2,4-tolylene diisocyanate of formula (8), 2,6-tolylene diisocyanate of formula (9), and 4,4′-diphenylmethane diisocyanate of formula (10). In addition, specific compounds of diol include polytetramethylene ether glycol of formula (11), polypropylene glycol of formula (12), and polyester polyol of formula (13). Here, n and m represent integers, and R, R1, and R2 are hydrocarbon. Moreover, specific compounds of crosslinking agent include 1,4-butylene glycol of formula (14), bis-2-hydroxy-ethoxy-benzene of formula (15), 4,4′-methylenebis(2-chloroaniline) of formula (16), and diethyl toluene diamine of formula (17).

[0019] In the present invention, polyester-type polyurethane thermoplastic elastomer is more preferable from the viewpoint of improvement in the physical properties of the material due to reaction or interaction when mixed with a metal compound.

[0020] The commercially available products of polyurethane-type thermoplastic elastomer include Elastollan ET500 type, ET600 type, or C type, particularly ET595, ET598, ET690, ET195, ET697, C60D, C98A, C95A, C90A, and so on from Takeda Badische Urethane Industries Ltd.

[0021] A metal compound used in the present invention is one or more kinds of metal compounds selected from the group consisting of alkali metal compound, alkaline-earth metal compound, or other metal compounds.

[0022] In the metal compound according to the present invention, an alkali metal, an alkaline-earth metal, and other metals beside titanium may also be used as a metal component. For instance, such other metals include lithium, sodium, magnesium, calcium, strontium, barium, aluminum, zinc, copper, nickel, cobalt, iron, and neodymium. According to the present invention, metal oxides or metal hydroxides containing these metals are preferred. Specific examples include magnesium oxide, zinc oxide, calcium carbonate, magnesium hydroxide, calcium hydroxide and the like. In the present invention, magnesium oxide, zinc oxide, and magnesium hydroxide can be conveniently used. The effects of the present invention cannot be derived from mixing titanate oxide, however.

[0023] The mixing ratio of polyurethane-type thermoplastic elastomer and a metal compound is preferably 0.1 to 30 parts by weight metal compound for 100 parts by weight polyurethane-type thermoplastic elastomer, and more preferably, 1 to 15 parts by weight metal compound, and particularly 5 to 15 parts by weight metal compound for 100 parts by weight polyurethane-type thermoplastic elastomer. When the metal compound mixed is less than 0.1 part by weight, the effect of mixing the metal compound diminishes so that the effect of improved resilience and improved scratch resistance cannot be achieved. On the other hand, when the metal compound exceeds 30 parts by weight, the viscosity of the cover material increases, thereby degrading moldability, and the cover material becomes too hard so that the shot feel becomes hard.

[0024] A cover material according to the present invention is a product obtained by heating and mixing polyurethane-type thermoplastic elastomer with a metal compound. For its production, a single-shaft extruder or a two-shaft extruder generally used for molding of resin may be used. In this case, it is preferred to effect the mixing normally with the cylinder temperature at 150° C. to 280° C. When the cylinder temperature is below 150° C., polyurethane-type thermoplastic elastomer does not melt sufficiently so that dispersion of the metal compound becomes poor, and no reaction would take place. On the other hand, when the cylinder temperature exceeds 280° C., polyurethane-type thermoplastic elastomer becomes degraded by heat, resulting in degradation of its physical properties.

[0025] Polyurethane-type thermoplastic elastomer and the metal compound can be added at the same time, or the metal compound may be added after polyurethane-type thermoplastic elastomer has melted. In addition, the residence time in the extruder can be appropriately adjusted so as to allow the reaction by the metal compound to progress by a prescribed amount.

[0026] In the present invention, thermoplastic resin and/or thermoplastic elastomer other than polyurethane-type thermoplastic elastomer as a polymer component in the cover material may be used at less than 50 percent by weight. Here, the possible thermoplastic resins include general-purpose resins such as ionomer resin, polyethylene, polypropylene, polystyrene, ABS (acrylonitrile-butadiene-styrene) resin, methacryl resin, polyethylene terephthalate, ACS (acrylonitrile-chlorinated polyethylene-styrene) resin, and polyamide, but ionomer resin is particularly preferred.

[0027] One example of an ionomer resin is a binary copolymer of α-olefin and an α, β-unsaturated carboxylic acid having 3 to 8 carbon atoms, where at least a portion of its carboxyl group is neutralized with metal ions. Moreover, another example is a ternary copolymer of α-olefin, an α, β-unsaturated carboxylic acid having 3 to 8 carbon atoms, and α, β-unsaturated carboxylate having 2 to 22 carbon atoms, which is obtained by having at least a portion of its carboxyl group neutralized with metal ions. When the base polymer of ionomer resin is a binary copolymer of α-olefin and an α, β-unsaturated carboxylic acid having 3 to 8 carbon atoms, the composition ratio is preferably 80 to 90 percent by weight α-olefin and 10 to 20 percent by weight α, β-unsaturated carboxylic acid.

[0028] When the base polymer is the ternary copolymer of α-olefin, an α, β-unsaturated carboxylic acid having 3 to 8 carbon atoms, and α, β-unsaturated carboxylate having 2 to 22 carbon atoms, the composition ratio is preferably 70 to 85 percent by weight α-olefin, 5 to 30 percent by weight, and more preferably, 12 to 20 percent by weight α, β-unsaturated carboxylic acid, and 10 to 25 percent by weight α, β-unsaturated carboxylate. In addition, these ionomer resins preferably have a melt index (MI) of 0.1 to 20, more preferably of 0.5 to 15. Resilience can be improved by setting the carboxylic acid content or the carboxylate content in the above range.

[0029] As α-olefin described above, for instance, ethylene, propylene, 1-butene, 1-pentene and the like are used, while ethylene is particularly preferred. As the α, β-unsaturated carboxylic acid having 3 to 8 carbon atoms described above, for instance, an acrylic acid, a methacrylic acid, a fumaric acid, a maleic acid, a crotonic acid and the like are used, while acrylic acid and methacrylic acid are particularly preferred. In addition, as unsaturated carboxylate, for instance, methyl, ethyl, propyl, n-butyl, isobutyl ester and the like of acrylic acid, methacrylic acid, fumaric acid, maleic acid and the like are used, while acrylate and methacrylate are particularly preferred. The possible metal ions for neutralizing at least a portion of the carboxyl group of the above-described copolymer of α-olefin and an α, β-unsaturated carboxylic acid or the ternary copolymer of α-olefin, an α, β-unsaturated carboxylic acid, and α, β-unsaturated carboxylate include sodium ions, lithium ions, zinc ions, magnesium ions, potassium ions and the like.

[0030] According to the present invention, when mixing polyurethane-type thermoplastic elastomer with thermoplastic resin and/or thermoplastic elastomer, these polymer components may be mixed first and then a metal compound may be added and mixed, or the polymer components and the metal compound can be mixed at the same time. In addition, polyurethane-type thermoplastic elastomer and the metal compound may be mixed in advance, and thereafter, thermoplastic resin and the like may be mixed. The heating and mixing process is normally performed using an internal mixer such as a kneading two-shaft extruder, a Banbury, and a kneader, and at a temperature of 150 to 260° C., for instance.

[0031] Moreover, the cover according to the present invention has a Shore D hardness of 30 to 60, preferably 40 to 60, and more preferably 44 to 57, when measured in the state of the ball. When the Shore D hardness is less than 30, the cover becomes too soft such that the initial velocity of the ball becomes low. Conversely, when the Shore D hardness exceeds 60, the spin rate upon striking the ball with a short iron and the like becomes small.

[0032] In addition, according to the present invention, for the cover composition, a filler such as barium sulfate, and titanium dioxide, and other additives such as a dispersing agent, an antioxidant, an ultraviolet absorber, a photo-stabilizer, a fluorescent material or a fluorescent brightening agent, and so on may be blended besides the resin as the main component according to need in such a manner that the desired characteristics of the golf ball cover is not lost.

[0033] In the present invention, a thread-wound core or a core for a solid ball such as a two-piece, a three-piece and the like is used as the core, and it may be adopted either for a thread-wound ball or to a solid ball. The core for a solid ball is formed by a crosslinked product of a rubber composition, and as a rubber component for the rubber composition, a butadiene rubber having a cis-1,4-structure is appropriately used as the base material. It is noted that, besides the butadiene rubber, natural rubber, styrene-butadiene rubber, isoprene rubber, chloroprene rubber, butyl rubber, ethylene-propylene rubber, ethylene-propylene diene rubber, acrylonitrile rubber and the like, for instance, may be blended at 40 parts by weight or below per 100 parts by weight of the rubber component.

[0034] An example of a crosslinking agent used for the rubber composition is metal salt of α, β-ethylenic unsaturated carboxylic acid produced by reacting an α, β-ethylenic unsaturated carboxylic acid such as an acrylic acid and a methacrylic acid with a metal oxide such as zinc oxide during preparation of the rubber composition. Other examples are metal salt of an α, β-ethylenic unsaturated carboxylic acid such as zinc acrylate and zinc methacrylate, a polyfunctional monomer, N, N′ phenylbis maleimide, sulfur and the like that are usually used as a crosslinking agent. In particular, metal salt of an α, β-ethylenic unsaturated carboxylic acid, and particularly zinc salt, is preferred. For instance, when metal salt of α, β-ethylenic unsaturated carboxylic acid is used, it is preferably blended at 20 to 40 parts by weight per 100 parts by weight of the rubber component. On the other hand, when α, β-ethylenic unsaturated carboxylic acid is made to react with a metal oxide during preparation of the rubber composition, 15 to 30 parts by weight α, β-ethylenic unsaturated carboxylic acid and 15 to 35 parts by weight metal oxide such as zinc oxide per 100 parts by weight α, β-ethylenic unsaturated carboxylic acid are preferably blended.

[0035] As a filler used for the rubber composition, one or more kinds of inorganic powders such as barium sulfate, calcium carbonate, clay, zinc oxide and the like, for instance, may be used. The amount of these fillers is preferably in a range of 5 to 50 parts by weight per 100 parts by weight of the rubber component.

[0036] In addition, a softening agent, liquid rubber or the like may be appropriately mixed for the purpose of improving workability and adjusting the hardness, or an antioxidant may be appropriately added.

[0037] As a crosslinking initiator, for instance, organic peroxide such as dicumyl peroxide and 1,1-bis(t-butyl peroxy) 3,3,5-trimethyl cyclohexane is used. The amount of such crosslinking initiator is preferably 0.1 to 5 parts by weight, and particularly 0.3 to 3 parts by weight per 100 parts by weight of the rubber component.

[0038] In the present invention, the core can be formed as a single layer or as multiple layers having different characteristics in specific gravity, hardness, and so on. In this case, the formulation of the core is not limited to the description given above.

[0039] The core is produced by mixing the above-described blending materials with a roll, a kneader, a Banbury and the like and vulcanizing the mixture using a mold at 145° C. to 200° C., preferably at 150° C. to 165° C., under pressure for 15 to 45 minutes. An adhesive may be applied to the surface of the core produced, or the surface of the core may be roughened so as to improve adhesion between the core and the cover.

[0040] The diameters of the thread-wound core and the solid core are designed in a range of 36.8 to 41.4 mm, and more preferably, in a range of 37.6 to 40.8 m. When the diameter is less than 36.8 mm, the cover layer becomes thick, which reduces resilience. On the other hand, when the diameter exceeds 41.4 mm, the cover layer becomes too thin so that the molding becomes difficult.

[0041] The molding of the cover according to the present invention can be performed using a known method. The cover composition may be shaped into hemispherical half shells in advance, and a core is enwrapped with two of these half shells, and then, is molded under pressure for one to five minutes at 130° C. to 170° C. Otherwise, a method may be employed in which the cover composition is directly formed by injection molding on the core to enwrap the core. A thickness of the cover is 0.7 to 3.0 mm, and preferably is 1.0 to 2.5 mm. When the thickness is smaller than 0.7 mm, a crack in the cover is likely to occur after repeated striking of the ball, whereas the shot feel becomes unsatisfactory when the thickness is greater than 3.0 mm. In addition, upon molding the cover, numerous dimples are formed on the surface of the golf ball according to need. Normally, a paint finish and a marking stamp are provided to the present golf ball in order to improve its aesthetic appearance and its product value before the golf ball is introduced to the market.

[0042] Moreover, the cover according to the present invention may be formed as a single-layer cover or as a multiple-layered cover.

[0043] Furthermore, the present golf ball is normally designed with a ball diameter in a range of 42.67 to 43.00 mm and a ball weight of 45.00 to 45.93 grams.

EXAMPLES EXAMPLES 1 to 7 Comparative Examples 1 to 4

[0044] (1) Production of inner core

[0045] A rubber composition for the core of the formulation shown in Table 1 was kneaded and was heat-pressed for 25 minutes at 155° C. within a mold to form a core having a diameter of 39 mm. TABLE 1 Parts by Formulation weight Polybutadiene*¹ 100 Zinc acrylate 25 Zinc oxide 6 Dicumyl peroxide*² 0.5 Sulfur compound*³ 0.5 Deformation(mm) of solid 4.9 core (under load of 10 to 130 Kg)

[0046] (2) Preparation of cover composition

[0047] The blending materials for a cover shown in Table 2 were mixed using a two-shaft kneading extruder to produce pellet-like composition for the cover. The extruding conditions were as follows:

[0048] Screw diameter: 45 mm

[0049] Screw speed: 200 rpm

[0050] Screw L/D: 35

[0051] The formulation was heated at 230° C. in the position of the die of the extruder.

[0052] Hemispherical half shells were formed using the cover composition obtained, and the core obtained as described above was enwrapped with two of these half shells. Then, the enwrapped core was molded by press-heat compression, and paint was applied on the surface, thus producing a golf ball having a diameter of 42.8 mm and a weight of 45.5 grams.

[0053] A golf ball thus produced was evaluated for its cover hardness (Shore D hardness), its spin rate as being indicative of its flying performance, and its shot feel. The result of evaluation is shown in Table 2.

[0054] Moreover, the evaluation of physical properties of a ball was performed in the following manner.

[0055] (i) Shore D hardness

[0056] Using a spring-type hardness tester Shore D type defined in ASTM D-2240, the outer surface of a golf ball (in a portion other than dimples) was measured five times. The average value is indicated.

[0057] (ii) Spin performance (spin maintenance)

[0058] A sand wedge club was attached to a swing robot manufactured by True Temper, and a golf ball was struck at a head speed of 20 m/s. A mark provided on the golf ball struck was photographed successively, and the spin rate was derived. The measurements were performed under a normal dry condition and under the wet condition in which the ball and the club face were wetted with water. The spin maintenance is defined as a value of (spin rate when wet)/(spin rate when dry)×100.

[0059] (iii) Scratch resistance

[0060] A commercially available pitching wedge was attached to a robotic machine, and a ball was struck in two places, one strike in each place, at a head speed of 36 m/s. The two struck locations were observed and evaluated in five grade levels.

[0061] 5: Little scratch remains on the ball surface but is barely noticeable

[0062] 4: Some scratch remains on the ball surface with little fuzz observed

[0063] 3: Scratch clearly defined on the ball surface with fuzz somewhat apparent

[0064] 2: Ball surface being a little scraped, with apparent fuzz

[0065] 1: Ball surface being scraped to some degree, with conspicuous fuzz

[0066] (iv) Resilience

[0067] A strike speed of a golf ball is measured when a swing robot strikes the golf ball at a speed of 45 m/s, and the relative value in relation to comparative example 1 was expressed by index. The greater the number, the better the resilience indicated. TABLE 2 Examples Comparative examples 1 2 3 4 5 6 7 1 2 3 4 Cover ET155D *1 100 100 100 100 formulation ET195 *2 100 100 100 C95A *3 100 100 100 Hytrel 4767 *4 100 Zinc oxide *5 1 5 1 10 1 15 Magnesium *6 5 5 oxide Titanate oxide *7 2 2 2 2 2 2 2 2 2 2 2 Pigment powder 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 Ball Shore D hardness 55 57 57 46 49 44 49 55 46 44 47 characteristics Spin maintenance 75 74 74 82 80 84 82 75 82 84 58 Scratch resistance 4 5 5 4 5 4 5 3 2 2 2 Resilience 102 105 106 104 107 103 107 100 103 103 102

[0068] Table 2 shows the results of the measurements of the golf balls of examples 1 to 7 and comparative examples 1 to 4.

[0069] Comparative examples 1 to 3 employ polyurethane-type thermoplastic elastomer, but no metal compound (other than titanate oxide) is mixed so that the scratch resistance proves inferior. In addition, comparative example 4 utilizes a mixture of polyester-type thermoplastic elastomer and a metal compound, and its scratch resistance proves inferior.

[0070] The examples of the present invention utilize the mixture of polyurethane-type thermoplastic elastomer and metal oxides (zinc oxide in all of examples 1, 2, 4, 5, 6, and 7; magnesium oxide in example 3) so that the spin maintenance, the scratch resistance, and the resilience are generally superior to those of the comparative examples.

[0071] As described above, the present invention mixes a metal compound (other than titanate oxide) with polyurethane-type thermoplastic elastomer so that it is speculated that the metal compound interacting or reacting with the polyurethane-type thermoplastic elastomer forms pseudo-crosslinking between main chains of polymers, thereby improving the strength of the material as a result. When such material is used for a cover of a golf ball, the spin characteristic, the resilience, and particularly the scratch resistance of the golf ball can be improved.

[0072] Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims. 

What is claimed is:
 1. A golf ball comprising a core and a cover, wherein a material of the cover is obtained by mixing polyurethane-type thermoplastic elastomer with a metal compound (other than titanate oxide).
 2. The golf ball according to claim 1, wherein said metal compound is an oxide or a hydroxide of a metal selected from the group consisting of zinc, sodium, magnesium, lithium, neodymium, copper, calcium, aluminum, iron, and barium.
 3. The golf ball according to claim 2, wherein said metal compound is zinc oxide.
 4. A golf ball comprising a cove and a cover, wherein a material of the cover is obtained by mixing polyurethane-type thermoplastic elastomer at 100 parts by weight with a metal compound (other than titanate oxide) at 5 to 15 parts by weight.
 5. The golf ball according to claim 4, wherein a hardness of the cover covering a core is 30 to 60 in Shore D hardness. 